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\begin{equation*}\frac{\Delta^2F(P_0)}{\Delta P_i^2}=\frac{\Delta F'(P_0)}{\Delta P_i}=\frac{\frac{\Delta F(P_1)}{\Delta P_i}-\frac{\Delta F(P_0)}{\Delta P_i}}{\Delta P_i},\,\!\end{equation*}
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